Accompanying increased traffic on the Internet, a larger capacity trunk-line optical communication system is demanded and, research and development is being conducted for an optical transceiver capable of transmitting signals of over 100 [Gbit/s] per wavelength. As bit rate per wavelength increases, deterioration of the signal quality becomes greater due to a decrease of the optical signal-to-noise ratio (OSNR) resistance, waveform distortion caused by wavelength dispersion, polarization mode dispersion, or nonlinear effect, etc., of a transmission line.
For this reason, in recent years, a digital coherent reception system is being studied that has such OSNR resistance and waveform distortion resistance (see, e.g., Japanese Patent Laid-Open Publication No. 2009-212994). As compared with a conventional system of direct detection by assigning the on/off of the light intensity to a binary signal, the digital coherent reception system extracts light intensity and phase information by the coherent receiving system. The extracted intensity and phase information is quantized by an analog-to-digital converter (ADC) and demodulated by a digital signal processing circuit.
The conventional technology described above, however, has a problem in that the reception quality in the digital coherent receiver deteriorates due to skew (delay time difference) between signals of each channel. Factors causing the skew caused between the signals of each channel are thought to be individual differences, etc., between constituent elements such as electric lines, optical hybrid circuits, opto-electronic converters, ADCs, etc., that make up a path of a channel, upstream from a digital processing unit.
The signals included in signal light are sampled, for example, at the ADCs with same phase timing and therefore, if there is skew between the signals of each channel, the sampling point deviates from the optimal point, resulting in deteriorated quality of the signal reproduced at the digital signal processing circuit.
While it is conceivable to perform sampling with different timings for the signals included in the signal light, a configuration must be disposed for controlling each sampling timing, resulting in a problem of an increased scale of the circuit.